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Process Management-Process Migration
- 2. Process Migration ο Relocationof a process from its current location to another node. ο Process may be migrated ν either before it start executing on its source node ο known as non-preemptive process migration. ν or during the course of its execution ο Known as preemptive process migration.
- 3. Process Migration • Selecting a process to be migrated • Selecting the destination node • Suspending the process • Capturing the process state • Sending the state to the destination • Resuming the process • Forwarding future messages to the destination Process P1 : : : : Execution suspended Source Site Destination Site Execution Resumed : : : : Process P1 Transfer of control Time Freezing time
- 4. Desirable features of a good process migration mechanism ο ο ο ο ο ο Transparency Minimal Interference Minimal Residual Dependencies- Efficiency Robustness Communication between coprocesses of a job
- 5. Transparency Level of transparency:(Location Independent)ν ν Object Access Level System Call & Interprocess Communication level
- 6. Minimal Interference ο Migration of a process should cause minimal interference of progress of the process involved. ο Achieve by minimizing the freezing time of the process being migrated. ν Freezing time is the time period for which the execution of the process is stopped for transferring its information to the destination node.
- 7. Minimal Residual Dependencies ο The process migrated should not depend on any factor with the previous process.
- 8. Efficiency ο Minimum time required for migrating a process. ο Minimum cost of locating an object. ο Minimum cost of supporting remote execution once the process is migrated.
- 9. Robustness ο Except the failure of a node on which a process is currently running should not affect the accessibility or execution of process.
- 10. Communication between coprocessors of a job ο To reduce communication cost, it is necessary that coprocesses are able to directly communicate with each other irrespective of their locations.
- 11. Process Migration Benefits • Better response time and execution speed-up –Dynamic load balancing among multiple nodes • Higher throughput and Effective resource utilization –Migrating I/O and CPU-bound processes to file and cycle servers. • Reducing network traffic –Migrating processes closer to the resources they are using most heavily. • Improving system reliability –Migrating processes from a site in failure to more reliable sites –Replicating and migrating critical processes to a remote.
- 12. Process Migration Mechanisms Four major activities 1. Freezing the process on its source node and restarting it on its destination node. 2. Transferring the process’s address space from its source node to its destination node. 3. Forwarding messages meant for the migrant process. 4. Handling communication between cooperating processes that have been separated as a result of process migration.
- 13. Cont… : Mechanisms for Freezing and Restarting a Process ο Freezing the process: ν The execution of the process is suspended in the source node and the state of information is transferred to its destination node and from there it is restarted. ο Issues: ν ν ν ν Immediate and delayed blocking of the process Fast and slow I/O operations Information about open files Reinstating the process on its Destination node
- 14. Cont… : Reinstating the process on its Destination Node ο On the destination node, an empty process state is created.
- 15. Cont… ο Once all the state of the migrating process has been transferred from the source to destination node and copied into the empty state, new copy of the process is unfrozen and old copy is deleted. ο The process is restarted on its destination node in whatever state it was in before being migrated.
- 16. Address Space Transfer Mechanisms ο The migration of a process involves the transfer of ν ν Process’s state Process’s address space from the source node to the destination node.
- 17. Address Space Transfer Mechanisms • Since the target process for migration is preempted , its state information must also be transferred. • State information consists of computation state and communication state . • The computation state is the information necessary to save and restore a process at the remote site and • communication state is the status of process communication links and messages in transit.
- 18. Process State consists of • CPU registers –Captured upon a freeze • Address space –Difficult to restore pointers • I/O state: –Fast I/O Operations •Completed before a process migration –Durable I/O Operations like files and user interactions •Difficult to carry files in use and to freeze/restore system calls. •Necessity to maintain a connection with I/O established at the source node.
- 19. Cont… ο Process address space ν ν ν code, data and program stack
- 20. Cont… ο Mechanisms for address space transfer: ν ν ν Total freezing Pre-transferring Transfer on reference
- 21. Process Migration Address Transfer Mechanisms Transfer of address space Source node Destination node Suspended Migration decision resumed Freezing time Transfer of address space Source node Destination node Suspended Migration decision resumed Freezing time On-demand transfer Source node Destination node Suspended Migration decision resumed Freezing time Total Freezing Pretransferring Transfer-on-reference Merits: easy implementation Demerits:long delay time Merits: freezing time reduce Demerits:total time Merits: quick migration Demerits: large memory latency
- 22. Cont… : Total Freezing ο A process’s execution is stopped while its address space is being transferred. ο Disadvantage: ν Process is suspended for long time during migration, timeouts may occur, and if process is interactive, the delay will be noticed by the user.
- 23. Cont … : Pretransferring ο Also known as precopying. ο The address space is transferred while the process is still running on the source node. ο After the transfer, the modified pages are picked up.
- 24. Cont … ο Reduces the freezing time of the process but it may increase the total time for migrating due to the possibility of redundant page transfers.
- 25. Cont… : Transfer on Reference ο The process address space is left behind on its source node, and as the relocated process executes on its destination node. ο Attempts to reference memory page results in the generation of requests to copy in the desired blocks from their remote location. ο A page is transferred from its source node to its destination node only when referenced.
- 26. Cont… ο Very short switching time of the process from its source node to its destination node.
- 27. Cont… ο Mechanisms: ν ν ν ν Mechanism of resending the message Origin site mechanism Link traversal mechanism Link update mechanism
- 28. Process Migration Message Forwarding Mechanisms Sender Origin Dest 1 Dest 2 Receiver Migrate Migrate again Resend Resend again Sender Origin Dest 1 Dest 2 Receiver Migrate Migrate again Resending messages Ask origin site SendSend Forward
- 29. Process Migration Message Forwarding Mechanisms (Cont’d) Sender Origin Dest 1 Dest 2 Receiver Migrate Migrate again Link Update Send Sender Origin Dest 1 Dest 2 Receiver Migrate Migrate again Link traversal Link Link Send Forward Forward Send New location Send New location Send Current location
- 30. Cont… : Mechanisms of resending the message ο Messages of are returned to the sender as not deliverable or are simply dropped, with the assurance that the sender of the message is storing a copy of the data and is prepared to retransmit it.
- 31. Cont… ο Disadvantage: ν The message forwarding mechanism of process migration operation is nontransparent to the processes interacting with the migrant process.
- 32. Cont… : Origin Site Mechanism ο Each site is responsible for keeping information about the current locations of all the processes created on it. ο Messages for a particular process are always first sent to its origin site. ο The origin site then forwards the message to the process’s current location.
- 33. Cont… ο Disadvantage: ν site will disrupt the messageFailure of the origin forwarding mechanism.
- 34. Cont… : Link Traversal Mechanism ο To redirect message, a message queue for the migrant process is created on its source node. ο A forwarding address known as link is left at the source node pointing to the destination of the migrant process. ο Components of link, one is unique process identifier and second is last known location.
- 35. Cont… ο Disadvantage: ν Several link may have to be traversed to locate a process from a node and if any node in chain of link fails, the process cannot be located.
- 36. Cont… : Link Update Mechanisms ο Link update message ν ν Tells the new address of each link held by the migrant process. Acknowledged for synchronization purposes.
- 37. Mechanisms for Handling Coprocesses ο To provide efficient communication between a process and its sub processes which might have been migrated on different nodes. ο Mechanisms : ν ν Disallowing separation of coprocesses. Home node or origin site concept.
- 38. Cont… : Disallowing Separation of coprocesses ο Easiest method of handling communication between coprocesses is to disallow their separation. ο Methods : ν ν By disallowing the migration of processes that wait for one or more of their children to complete. By ensuring that when a parent process migrates,its children process will be migrated along with it.
- 39. Cont… ο Disadvantage: ν It does not allow the use of parallelism within jobs.
- 40. Cont… : Home node or Origin Sites Concept ο Used for communication between a process and its sub process when the two are running on different nodes. ο Allows the complete freedom of migrating a process or its sub process independently and executing them on different nodes of the system.
- 41. Cont… ο Disadvantage: ν All communication between a parent process and its children processes take place via the home node. ο traffic and the communication cost increaseThe message considerably.
- 42. Advantages of Process Migration ο Reducing average response time of processes ν To reduce the average response time of the processes, processes of a heavily loaded node are migrated to idle or underutilized nodes. ο Speeding up individual jobs ν ν ν A migration of job to different node is done and execute them concurrently. Migrate a job to a node having a faster CPU or to a node at which it has minimum turnaround time. More speed up more migration cost involved.
- 43. Cont… ο Gaining higher throughput ν Process migration facility may also be used properly to mix I/O and CPU-bound processes on a global basis for increasing the throughput of the system. ο Utilizing resources effectively ν Depending upon the nature of a process, it can be migrated to suitable node to utilize the system resource in the most efficient manner.
- 44. 59 Cont… ο Reducing network traffic ν Migrating a process closer to the resources it is using most heavily. ο Improving system reliability ν Migrate a copy of a critical process to some other node and to execute both the original and copied processes concurrently on different nodes. ο Improving system security ν A sensitive process may be migrated and run on a secure node.